() The government shutdown is causing serious headaches for air travelers across the country as it stretches into its second week.

On Wednesday, hundreds of flights were delayed and dozens canceled as airports struggled to keep operations going amid the government shutdown, according to data from FlightAware.

Though tens of thousands of TSA workers and air traffic controllers are considered essential workers and expected to clock in during a shutdown with no pay, sick days and walkouts could worsen already existing staffing struggles at airports across the United States.

Late Tuesday night, an air traffic controller shortage triggered a ground stop at Nashville’s airport, which had already announced earlier in the day it would be reducing the number of flights in and out until further notice.

Boston Logan International Airport and Chicago O’Hare International Airport also experienced ground delays due to staffing issues, according to local affiliate WGN.

Hollywood Burbank Airport’s control tower was unstaffed for more than six hours on Monday, leading to cancellations and hourslong delays.

“We get here, no luggage. We couldn’t talk to anybody,” traveler Vicki Davis said Monday. “And it’s been heck trying to get in touch with somebody with the luggage.”

Derek Bodkin’s flight was delayed from departing the Los Angeles-area airport that same day: “I have to wait two hours to depart from here, and I’ll miss my connector. So I’ve asked my wife to drive up to San Francisco.”

The U.S. had already faced a serious shortage of air traffic controllers, and the Federal Aviation Administration released plans to address a shortfall of nearly 3,000 controllers earlier this year.

Transportation Secretary Sean Duffy said his department has seen a small uptick in workers calling out sick during the shutdown, which could lead to a ripple effect for travelers.

TSA told it hasn’t experienced delays in operations or call-outs, but some airports are working through contingency plans and offering free food vouchers and parking to incentivize their employees to continue coming in.